Choosing Direct Hot Air or Hydronic Marine Heating

Direct Hot Air or Hydronic Marine Heating

Before you embark on a project to heat your boat, you should think about whether you want to use a direct air heater or a hydronic system. Each has advantages and disadvantages. It is important to decide which way to go up front because once you install an air heater the possible benefits of a hydronic system potentially doing air heating and hot water in one system are lost.

Direct Air Heating

A direct air heating system consists of a diesel air heater that generates hot air directly, which is then ducted around the boat. We offer diesel air heater kits at different sizes and price points to suit all boats. You can read more about this specific option here. If you are totally happy with your boat’s hot water system (quantity, availability, system age/condition, reheat times) then direct air heating is generally the way to go.

Hydronic Air Heating

Hydronic air heating runs off a diesel hydronic furnace that heats coolant, which is circulated in the boat to fan heads and usually include water heating as well. If your boat cannot fit air heating duct, or you want more hot water, hot water without running the engine, solar hot water etc. then hydronic has definite advantages.

Hydronic Systems


  • Hydronic systems can provide hot air and hot water in one system. This facilitates hot water when engine heat is not available, which can be a real advantage, particularly in sailing boats where the engine is not run often.
  • Existing hot water cylinders can often be incorporated into the system by disconnecting them from the engine.
  • Reheat times for hot water cylinders from a diesel furnace are usually shorter than from 240v element (5-12kW versus 1-3kW), this allows more people to have a shower or smaller hot water cylinders to be installed.
  • Hydronic systems allow the incorporation of Electric Solar hot water, allowing surplus solar or wind power to be diverted to the hot water tank.
  • Hot coolant piping can be easier to run in a boat than ducting (it has a smaller diameter so will wiggle through tight spaces or can sit in the bilge space).
  • The temperate in each area of a boat can be individually managed using the Dieselheat hydronic thermostat.
  • Getting outlets in hard to reach spots can be achievable.
  • A combined hydronic and hot water system is usually more compact than separate systems.


  • Generally hydronic systems cost more than direct air systems – but they do incorporate endless hot water…
  • Heat modulation (controlling the air temperature) can be trickier unless specific thermostats are installed. 
  • Systems take longer to get started and warm up (when compared to air heaters), especially when heating water as well.
  • Systems generally require a more fine tuning or balancing to get them working properly and installation time is normally longer as there are more parts and it is more complex.
  • Hydronic fan heads do not project the hot air as forcefully as a direct diesel heater, so the hot air does not spread out as much in the boat.
  • We use the quietest Kalori fan heads available, however, in some cases they are slightly noisier than ducted air heating because the fans are at the point of heating in the living space.

Direct Air Heating


  • Can be the quickest and cheapest systems to install/set up.
  • Has the fastest heat up time.
  • Outlet air velocity from hot air vents is faster, so the hot air tends to spread out better in the vicinity of the outlet.
  • Temperature control is easier in a main area as air heaters have thermostat built in. as outputs are more variable, air heaters modulate their output better than a fan head.


  • Does not resolve lack of hot water issues.
  • Temperature control in zones is generally not possible.
  • Larger diameter ducting takes more space and loses more heat than hydronic pipes.
  • Getting air distributed throughout the boat can be harder due to the ducting sizes.

In summary heating a boat can be done 2 ways, often dictated by hot water needs, space, budget etc. Either system requires some element of design and sizing of furnaces, ducting, pipe networks, controls etc. Dieselheat specialises in assisting boat owners to decide which system suits them best and in designing and assembling complete boat specific kits for either air or hydronic systems.


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